FIG. 1 shows the schematic arrangement of a magnetic disk drive 10 using a rotary actuator. A disk or medium 11 is mounted on a spindle 12 and rotated at a predetermined speed. The rotary actuator comprises an arm 15 to which is coupled a suspension 14. A magnetic head 13 is mounted at the distal end of the suspension 14. The magnetic head 13 is brought into contact with the recording/reproduction surface of the disk 11. The rotary actuator could have several suspensions and multiple magnetic heads to allow for simultaneous recording and reproduction on and from both surfaces of each medium.
An electromagnetic converting portion (not shown) for recording/reproducing information is mounted on the magnetic head 13. The arm 15 has a bobbin portion for holding a driving coil (not shown). A voice coil motor 19 as a kind of linear motor is provided to the other end of the arm 15. The voice motor 19 has the driving coil wound on the bobbin portion of the arm 15 and a magnetic circuit (not shown). The magnetic circuit comprises a permanent magnet and a counter yoke. The magnetic circuit opposes the driving coil to sandwich it. The arm 15 is swingably supported by ball bearings (not shown) provided at the upper and lower portions of a pivot portion 17. The ball bearings provided around the pivot portion 17 are held by a carriage portion (not shown).
A magnetic head support mechanism is controlled by a positioning servo driving system. The positioning servo driving system comprises a feedback control circuit having a head position detection sensor (not shown), a power supply (not shown), and a controller (not shown). When a signal is supplied from the controller to the respective power supplies based on the detection result of the position of the magnetic head 13, the driving coil of the voice coil motor 19 and the piezoelectric element (not shown) of the head portion are driven.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), remanent squareness (S), medium noise, i.e., signal-to-medium noise ratio (SMNR), and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium, and by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording density of high-density magnetic hard disk drives, and is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, media noise, including DC noise, must be minimized by suitable microstructure control.
According to the domain theory, a magnetic material is composed of a number of submicroscopic regions called domains. Each domain contains parallel atomic moments and is magnetized to saturation, but the directions of magnetization of different domains are not necessarily parallel. In the absence of an applied magnetic field, adjacent domains may be oriented randomly in any number of several directions, called the directions of easy magnetization, which depend on the geometry of the crystal. The resultant effect of all these various directions of magnetization may be zero, as is the case with an unmagnetized specimen. When a magnetic filed is applied, the domains most nearly parallel to the direction of the applied field grow in size at the expense of the others. This is called boundary displacement of the domains or the domain growth. A further increase in magnetic field causes more domains to rotate and align parallel to the applied field. When the material reaches the point of saturation magnetization, no further domain growth would take place on increasing the strength of the magnetic field.
The ease of magnetization or demagnetization of a magnetic material depends on the crystal structure, grain orientation, the state of strain, and the direction and strength of the magnetic field. The magnetization is most easily obtained along the easy axis of magnetization but most difficult along the hard axis of magnetization. A magnetic material is said to possess a magnetic anisotropy when easy and hard axes exist. On the other hand, a magnetic material is said to be isotropic when there are no easy or hard axes.
In a perpendicular recording media, magnetization is formed easily in a direction perpendicular to the surface of a magnetic medium, typically a magnetic layer on a suitable substrate, resulting from perpendicular anisotropy in the magnetic layer. On the other hand, in a longitudinal recording media, magnetization is formed in a direction in a plane parallel to the surface of the magnetic layer, resulting from longitudinal anisotropy in the magnetic layer.
A cross sectional view of a longitudinal recording disk medium is depicted in FIG. 2. A longitudinal recording medium typically comprises a non-magnetic substrate 20 having sequentially deposited on each side thereof an underlayer 21, 21′, such as chromium (Cr) or Cr-alloy, a magnetic layer 22, 22′, typically comprising a cobalt (Co)-base alloy, and a protective overcoat 23, 23′, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer 21, 21′, magnetic layer 22, 22′, and protective overcoat 23, 23′, are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy underlayer.
A perpendicular recording disk medium is shown in FIG. 3. It is similar to the longitudinal recording medium, but with the following differences. First, a perpendicular recording disk medium has soft magnetic underlayer 31 of an alloy such as Permalloy, which form predominantly a polycrystalline phase, instead of a Cr-containing underlayer typically used in a longitudinal recording media. Second, as shown in FIG. 3, magnetic layer 32 of the perpendicular recording disk medium comprises domains oriented in a direction perpendicular to the plane of the substrate 30. Also, shown in FIG. 3 are the following: (a) read-write head 33 located on the recording medium, (b) traveling direction 34 of head 33 and (c) transverse direction 35 with respect to the traveling direction 34.
Because a magnetic recording layer that is capable of perpendicular recording has domains (magnetic anisotropy) oriented in a direction perpendicular to the plane of the substrate, it is not capable of performing as a magnetic layer in a longitudinal recording medium. Similarly, because a magnetic recording layer that is capable of longitudinal recording has domains (magnetic anisotropy) oriented in a direction in the plane of the substrate, it is not capable of performing as a magnetic layer in a perpendicular recording medium. Besides, there exists no practically useable recording medium that is capable of working as a magnetic layer in both perpendicular and longitudinal recording media. An isotropic magnetic material is “theoretically” capable of working as a magnetic layer in both perpendicular and longitudinal recording media. However, an isotropic magnetic material, which lacks anisotropy, is never used in a magnetic recording medium as a magnetic recording layer because it lacks essential properties, such as high SMNR, of a magnetic recording layer.
A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Al—Mg) alloy. Such Al—Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture.
Other substrate materials have been employed, such as glass, e.g., an amorphous glass, glass-ceramic material which comprises a mixture of amorphous and crystalline materials, and ceramic materials. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks.
The underlayer and magnetic layer are conventionally sequentially sputter deposited on the substrate, typically by magnetron sputtering, in an inert gas atmosphere such as an atmosphere of pure argon. A conventional carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 Å thick.
When soft underlayers are fabricated by magnetron sputtering on disk substrates, there are several components competing to determine the net anisotropy of the underlayers: effect of magnetron field, magnetostriction of film and stress originated from substrate shape, etc. A seedlayer, which is optionally added as a layer lying in between the substrate and the underlayer, can often control anisotropy of the underlayer by promoting microstructure that exhibit either short-range ordering under the influence of magnetron field or different magnetostriction. A seedlayer could also alter local stresses in the soft underlayer.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SMNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by one or more underlying layers on which it is deposited. One form of a multiple underlayer containing perpendicular recording medium is a double-layer perpendicular recording medium that typically includes a substrate, a relatively thick soft underlayer (SUL), non-magnetic intermediate layer(s) and recording layer, in this order.
A “soft magnetic material” is a material that is easily magnetized and demagnetized. As compared to a soft magnetic material, a “hard magnetic” material is one that neither magnetizes nor demagnetizes easily. The problem of making soft magnetic materials conventionally is that they usually have many crystalline boundaries and crystal grains oriented in many directions. In such metals, the magnetization process is accompanied by much irreversible Block wall motion and by much rotation against anisotropy, which is usually irreversible. See Mc-Graw Hill Encyclopedia of Science & Technology, Vol. 5, 366 (1982). Mc-Graw Hill Encyclopedia of Science & Technology further states that the preferred soft material would be a material fabricated by some inexpensive technique that results in all crystal grains being oriented in the same or nearly the same direction. Id. Applicants, however, have found that “all grains” oriented in the same direction would be very difficult to produce and would not be the preferred soft material. In fact, applicants have found that very high anisotropy is not desirable.
It is important to reduce head-to-SUL spacing (HSS) and head-to-medium spacing (HMS) for high-density perpendicular recording. Smaller HSS and HMS allow efficient writing (recording) and reading, respectively. Using the thinnest possible intermediate, recording and protection layers also helps reduce HSS.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. This objective becomes particularly significant as the areal recording density increases. The areal density (Mbits/in2) is the recording density per unit area and is equal to the track density (TPI) in terms of tracks per inch times the linear density (BPI) in terms of bits per inch.
The increasing demands for higher areal recording density impose increasingly greater demands on flying the head lower because the output voltage of a disk drive (or the readback signal of a reader head in disk drive) is proportional to 1/exp(HMS). Therefore, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk to be positioned in closer proximity with an attendant increase in predictability and consistent behavior of the air bearing supporting the head.
It is a general trend that the surface of thin films gets rougher as film thickness increases. Since soft underlayer thickness is relatively thick (200–400 nm), the surface of the SUL tends to be rough, which contributes to the roughness of the magnetic recording media. When the surface of the magnetic recording media is rough, it is difficult to fly a head close to the surface of media. Therefore, reducing the surface roughness of the SUL is critical for reducing the surface roughness of the magnetic recording media and, thus, to reduce HMS.
Perpendicular recording media having a thick SUL are subjected to perpendicular anisotropy components in SUL originating from stress or magnetic anisotropy, thereby, providing energy to form stripe and ripple domains, which results in DC noise. The terms “stripe and ripple domains” are explained in references: K. Sin et al., IEEE Trans. Magn. 33, 2833 (1997) and N. Sato et al., J. Phys. Soc. Japn. 19, 1116 (1964), which are incorporated herein by reference. DC noise is noise originating from sources other than transition of recorded bits and, therefore, is independent of recording frequency.
This invention addresses the issues of surface roughness and DC noise of a high areal density, perpendicular magnetic recording medium by the use of an amorphous soft underlayer and their functional equivalents thereof.